Final answer:
The claim that personality traits are highly unstable across the adult lifespan is false. Studies show that the Big Five personality traits are relatively stable, with minor changes as people age.
Step-by-step explanation:
Extensive research on the Big Five personality factors, which include conscientiousness, agreeableness, neuroticism, openness, and extraversion, demonstrates that these traits are relatively stable over a person's lifetime. Notably, conscientiousness and agreeableness have been found to increase slightly with age, while neuroticism and extraversion tend to decline. Conclusively, multiple studies over time have validated the presence of the Big Five factors across cultures and suggest that these personality traits are influenced by genetics to a substantial extent. This stability and universal applicability of the Big Five factors across different populations underscore the overall consistency of personality traits through adulthood.